Date: Sat, 20 Aug 2022 08:20:00 +0000
To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: alicexbt <alicexbt@protonmail.com>
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Subject: [bitcoin-dev] joinstr: coinjoin implementation using nostr
Precedence: list

Hi Bitcoin Developers,

I have written a python script as proof of concept for the [coinjoin implem=
entation][1] using [nostr][2]. I used a lot of Python scripts created by ot=
hers in school, so it feels nice to offer something that could be useful to=
 others.

The implementation uses Bitcoin Core wallet and RPCs: `listunspent`, `getne=
waddress`, `scantxoutset`, `createpsbt`, `combinepsbt`, `finalizepsbt` and =
`sendrawtransaction`. It requires python-nostr library because nostr is use=
d for coordination between peers. Nostr is a decentralized network based on=
 cryptographic keypairs. It is not peer-to-peer however simple and scalable=
.

Every step is published as an event using a nostr relay and 5 peers coordin=
ate to create, sign and broadcast a coinjoin transaction.  I need to write =
a NIP that would be an alternative to blind signatures. Relay will share a =
random secret with clients for one round which should be present in output =
registration request although never gets published. If someone tries to reg=
ister an output without registering any inputs, request would not have the =
number initially shared with inputs so request would get rejected or publis=
hed as unverified. Relay would not be able to link inputs and outputs as th=
e number is same for all inputs in a round and they get registered at diffe=
rent times with new keys and IP address. Clients can use multiple relays at=
 the same time to avoid trusting one relay. This would result in different =
shared secret number but same process. If a relay tries to cheat, users wil=
l not sign the transaction and avoid using it in future.

Usage:

 1)Run `python coinjoin.py` and enter descriptor for one of the inputs.
 2)Script will check inputs for this round in every 30 seconds and register=
 a new adddress for output once 5 inputs are registered.
 3)Similar check happens every 30 seconds for outputs. Last peer should cre=
ate a PSBT.
 4)Unsigned PSBT will be printed and signed by wallet with `walletprocessps=
bt` RPC.
 5)Script will check signed PSBTs and last peer to sign should finalize coi=
njoin transaction once 5 signed PSBTs are received.
 6)Coinjoin transaction will be broadcasted and txid will printed.

Example:

```
List of utxos in wallet:

wpkh([53830dca/84'/1'/0'/0/0]02449be5fb74725255eeeb50eba930fa87705f21e99d13=
cd710cf2c1f21153c808)#x2hyyeg5

Enter descriptor for the input registration: wpkh([53830dca/84'/1'/0'/0/0]0=
2449be5fb74725255eeeb50eba930fa87705f21e99d13cd710cf2c1f21153c808)#x2hyyeg5

event id:  bcbbe62d75d99fed73f1e50ac58a38d1840b658951893e63c0322b378d7d56f0

```
```
tb1qhxrp4zl54ul0twtyz0gury5399q7z0kvqqrl6m registered for output

event id: 9449c9065bef356d21507a98f88b028b17fc1c49eb195c8d4420604fcaaef041
```
```
Unsigned PSBT: cHNidP8BAP1yAQIAAAAFtMaoJYcXvOG5L3Yaz3YyS7gIt4h5/zzOrRRS3hrV=
vwoAAAAAAP////+o83geaSm4L76KToIUl5MiZqLAUbIDJLq6DWrjP/3b8AEAAAAA/////zEF3CX=
IvVHpIa7No1s1yg+KtyOfXTRSyWnOdXMfzcDwAQAAAAD/////wMa4XAgnU+39Ien+KG9rYtv8bL=
MNYakmZyY/QFfwLRcAAAAAAP/////5M42ID6uLmQTb2tnFHnN7UMpnDD25uN8ZX7A+GNSM3QEAA=
AAA/////wV4xwEAAAAAABYAFLmGGov0rz71uWQT0cGSkSlB4T7MeMcBAAAAAAAWABSc0/FM6Hdb=
dxh10IJkYOklVFWqjnjHAQAAAAAAFgAUPSZKe/w6PT6qIF+WhL4wHaFymjd4xwEAAAAAABYAFMx=
0rxYlpPWB3NFry4Ctk2eVi/UNeMcBAAAAAAAWABSzc4xK0VTfvjK0MHXrAUFLYgYnOgAAAAAAAA=
AAAAAAAAAAAA=3D=3D

event id: 976744b38fa9343fb79e1b5215512ead6ee08e5890d79a201fc5b872f6de4eba
```
```
Signed PSBT: cHNidP8BAP1yAQIAAAAFtMaoJYcXvOG5L3Yaz3YyS7gIt4h5/zzOrRRS3hrVvw=
oAAAAAAP////+o83geaSm4L76KToIUl5MiZqLAUbIDJLq6DWrjP/3b8AEAAAAA/////zEF3CXIv=
VHpIa7No1s1yg+KtyOfXTRSyWnOdXMfzcDwAQAAAAD/////wMa4XAgnU+39Ien+KG9rYtv8bLMN=
YakmZyY/QFfwLRcAAAAAAP/////5M42ID6uLmQTb2tnFHnN7UMpnDD25uN8ZX7A+GNSM3QEAAAA=
A/////wV4xwEAAAAAABYAFLmGGov0rz71uWQT0cGSkSlB4T7MeMcBAAAAAAAWABSc0/FM6Hdbdx=
h10IJkYOklVFWqjnjHAQAAAAAAFgAUPSZKe/w6PT6qIF+WhL4wHaFymjd4xwEAAAAAABYAFMx0r=
xYlpPWB3NFry4Ctk2eVi/UNeMcBAAAAAAAWABSzc4xK0VTfvjK0MHXrAUFLYgYnOgAAAAAAAQBx=
AgAAAAG+qpMXZCy6tBuUlgo8JD0GVXKp60FkhwDeg2sF1fkFkwMAAAAA/f///wLo9wEAAAAAABY=
AFFfLA5xarC/w/SxeMDQ5tuXrYJLUWwMAAAAAAAAWABRfPf//hwMjHB4OKj87cU19XOSh7yOWAQ=
ABAR/o9wEAAAAAABYAFFfLA5xarC/w/SxeMDQ5tuXrYJLUAQhrAkcwRAIgOIhLoC5348U8YkEr4=
GU1K4yWskIOEXgW4Wsk/W2cR7ICIEJXqtOuDJ5CkwrSuwJLWtzab4dslbN3KuL/pyooMnOCASEC=
RJvl+3RyUlXu61DrqTD6h3BfIemdE81xDPLB8hFTyAgAAAAAACICA77Cnd6o3kr0yc+91eabpOn=
5igs/MUMbudNYSS6oyMWMGFODDcpUAACAAQAAgAAAAIAAAAAAFAAAAAAAAAAA

event id: 5846b6e6902f3c5a43496d7d9785ed62444aa74963f03c33d637d8b09ee7a139
```
```
Coinjoin tx: 75e490b10b15a6a0422f25ff66ad98ef70390c8fecaac02712705dce8cc356=
4b

event id: 9b5d4bf279b59e2b6e539e683fba83da72dce2b640360aa95db1b1400be93190
```

There are lot of things that could be improved and a few suggestions are in=
 the gist that described the [idea][3]. I would love read to any opinions a=
bout this experiment and will start working on creating an Android app for =
joinstr next week.

Credits:

- fiatjaf (Nostr)
- Andrew Chow (PSBT)
- Jeff Thibault (python-nostr)
- Existing coinjoin implmentations

[1]: https://github.com/1440000bytes/joinstr
[2]: https://github.com/nostr-protocol/nostr
[3]: https://gist.github.com/1440000bytes/1c305097b070c8374cc3b91f50314a45

/dev/fd0

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